lecture 5

Question 1

What are blood glucose levels?

Answer 1

• the blood glucose concentration is maintained within a narrow range
• the range is compatible with health and life
• normal range ~4.5 mM (60-90 mg/100mL)
• maintained at this level because glucose is critical to brain function
• increasing number of symptoms as level drops:
  
  • subtle neurological signs; hunger
  • Release of glucagon, epinephrine, cortisol
  • Sweating, trembling
  • Lethargy
  • Convulsions, coma (~30 mg/100mL)
  • Permanent brain damage (if prolonged, ~10mg/100mL)
  • death
• not just a matter of eating glucose we have to be able to use it effectively

Question 2

What happens after a meal?

Answer 2

• dietary glucose and amino acids enter the blood from the gastrointestinal tract
• dietary lipids are mainly packaged into chylomicrons which enter the blood via the lymphatic system (a network of vessels that drains excess fluid from interstitial spaces, filters out destroyed microorganisms and returns the fluid to the blood)

Question 3

What happens as blood glucose is secreted?

Answer 3

• insulin is secreted by the β cells of the pancreatic islets of Langerhans
• all actions of insulin in relation to glucose metabolism are aimed at lowering blood glucose to within the normal range
• insulin, released soon after meals, stimulates:
  
  • the storage of fuels
  • the synthesis of proteins
• insulin is only released with rising blood glucose so if you didn’t eat all day you wouldn’t get release

Question 4

What is the structure of insulin?

Answer 4

• A chain and B chain
• small protein
• peptide hormone
• the first protein product of the gene for insulin is unprocessed preproinsulin
• first protein product is not active
• chops off signal sequence at NH3 end
  
  • the signal sequence targets insulin to granules in the beta cells
• proinsulin
  
  • cleaved by particular proteases at two very distince points to release the C peptide
  • this results in formation of the active insulin
• mature insulin
  
  • formation of important disulphide linkages along the path of its formation
  • two between the chains and one within the A chain
  • strong covalent bonds

Question 5

What are Glucose transporters?

Answer 5

• GLUTs are membrane-bound proteins
• GLUTs mediate the movement of blood glucose into animal cells via the plasma membrane
• GLUT isoforms differ in their:
  
  • tissue expression
  • substrate specificty
  • kinetic characteristics
  • function
• do transport some other hexoses related to glucose
• GLUTs 1-5 are each single polypeptides of about 500 amino acid residues with 12 transmembrane domains
• non-polar amino acids in the membrane spanning domains
• more polar amino acids generally in hte aqueous environments
• they are arranged in the non-polar membrane in such a way that they create a polar core

Question 6

What is the function of GLUT4 in muscle cells (myocytes)?

Answer 6

• a process of ‘regulated exocytosis’
• glucose uptake is the rate-limiting step in its use by the cell
• failure of GLUT4 transport to the plasma membrane is an early feature of insulin resistance
• about 90% of insulin-stimulated glucose uptake occurs in muscle
• muscle is a big opportunity to increase or decrease blood glucose
• insulin doesn’t enter the cells
• insulin receptor is a dimer

1. glucose transporters “stored” within cell in membrane vesicles
2. when insulin interacts with its receptor, vesicles move to surface and fuse with the plasma membrane, increasing the number of glucose transporters in the plasma membrane
3. when insulin level drops, glucose transporters are removed from the plasma membrane by endocytosis, forming small vesicles
4. the smaller vesicles fuse with larger endosome
5. patches of the endosome enriched with glucose transporters bud off to become small vesicles, ready to return to the surface when insulin levels rise again
6. repeat

Question 7

What is the mechanism of insulin release?

Answer 7

• ↑ uptake and metabolism of glucose → ↑ ATP
• ↑ ATP inhibits the ATP-gated K+ channel
• the plasma membrane is depolarised (inside more +ve than outside)
• a voltage-gated Ca²⁺ channel opens and Ca²⁺enters the beta cell. Ca²⁺ is also released from intracellular stores in the ER
• ↑ Ca²⁺ triggers insulin release by exocytosis

Question 8

What are the three main tissues targeted by insulin release?

Answer 8

• the liver
• (the brain)
• adipose tissue
• muscle

Question 9

What are the outcomes of insulin actions in target tissues?

Answer 9

• ↑ uptake by tissues of glucose from blood
• ↑ storage of glucose in glycogen
• ↑ metabolism of glucose for energy
• ↑ synthesis of TAGs (from acetyl-CoA)
  
  • transported from the liver via VLDL and stored in adipose tissue

Question 10

What is glucagon?

Answer 10

• a peptide hormone, like insulin
• produced in the α cells of the pancreatic islet
• the hypothalamus responds to decreasing blood glucose levels by activation fof K+-ATP channels → autonomic nerve signalling to promote glucose secretion from α cells
• target target tissues are the liver (main), and adipose tissue
• acts quickly via a cell-surface receptor to activate cell signalling pathways

Question 11

What are the effects of glucagon on blood glucose?

Answer 11

• action on target enzyme
  
  • ↓ glycogen synthase
  • ↑ glycogen phosphorylase
  • ↓ pyruvate kinase
  • ↑ fructose 1,6-bisphosphatase
  • ↓ phosphofructokinase 1
  • ↑ triaglycerol lipase
• metabolic effect (tissue)
  
  • ↓ glycogen synthesis (liver)
  • ↑ glycogen breakdown (liver)
  • ↑ gluconeogenesis (liver)
  • ↑ gluconeogenesis
  • ↓ glycolysis (liver)
  • ↑ mobilisation of fatty acids (adipose)
• binding sites for glucagon have also been found in kidney, intestinal smooth muscle, brain

Question 12

What is Type 1 diabetes?

Answer 12

• diabetes mellitus
• absolute lack of insulin due to autoimmune destruction of β cells on pancreatic islets
• usually diagnosed in childhood
• no cure
• standard treatment involves self-monitoring of blood glucose and injection of insulin
• several genes have been associated with inherited type 1 diabetes
• symptoms if ‘uncontrolled’ (i.e. high blood glucose levels):
  
  • frequent urination, extreme thirst and dry mouth, hunger, sudden weight loss, lack of energy and blurred vision

Question 13

What is lost in diabetes?

Answer 13

• pancreatic β-cells are lost
• microscopic studies of pancreatic tissue show the relative loss of β-cells in type 1 and type 2 diabetes (T1DM and T2DM)
• the loss of β-cell mass is less in T2DM than T1DM
• in vitro and in vivo studies suggest that pro-inflammatory cytokines cause pancreatic β-cell death, especially in T1DM

Question 14

What is type 2 diabetes?

Answer 14

• heterogeneous:
  
  • varying levels of insulin production and release in the pancreas
  • varying levels of insulin resistance in the target tissues liver, adipose tissue and muscle
• the destruction of pancreatic β cell function increases as the T2DM progresses
• symptoms are very similar to those of T1DM

Question 15

What happens to muscle glucose metabolism in T1DM?

Answer 15

• failure of glucose uptake
• glycogen synthesis is inhibited
• glycogen stores are depleted
• protein metabolism
  
  • the uptake of amino acids and protein synthesis is inhibited
  • the normal inhibition of protein degradation is lost
  • muscle is degraded
  • consequence: increase in transport of nitrogen to the liver as the amino acid alanine → gluconeogenesis → worsening of hyperglycaemia

Question 16

What is adipose tissue metabolism in T1DM?

Answer 16

• failure of glucose uptake by the GLUTs
• increased lipolysis of triacylglycerols (TAGs), leading to increased levels of circulating free fatty acids
• transfer of free fatty acids to the liver for use as fuel (instead of glucose)

Question 17

What is liver glucose metabolism in T1DM?

Answer 17

• glycogenolysis (break down of glycogen stores) is favoured due to:
  
  • loss of activation of glycogen synthase by insulin
  • loss of inhibition of glycogen phosphorylase
• glycogenolysis contributes to increased blood glucose levels
• increased fatty acid oxidation leads to ketone body formation
• with the lack of insulin, high levels of ketone bodies plus high blood glucose lead to ‘diabetic ketoacidosis’

Question 18

What is diabetic ketoacidosis?

Answer 18

• no insulin
  
  • glucagon and other stress hormones released
  • breakdown of protein in muscle to amino acids, and of TAGs in adipose tissue to fatty acids
• blood glucose rises
  
  • exceeds the ability of the kidneys to retain glucose
  • glucose appears in urine, taking water with it
• increased urination
  
  • dehydration
• fats are metabolised
  
  • increased production of ketone bodies in the liver

Question 19

What is insulin resistance?

Answer 19

• affects the insulin’s target tissues: liver, adipose tissue and skeletal muscle
• insulin effects are ‘subnormal’
  
  • less glucose disposal in muscle
  • less suppression of endogenous glucose production in the liver
• it is thought that disruption of insulin receptor signalling pathways could contribute to insulin resistance
• T2DM occurs when there is:
  
  • insulin resistance in insulin target tissues PLUS
  • death and/or dysfunction of pancreatic β cells

Question 20

What are the three phases in the development of T2DM?

Answer 20

1. tissues develop insulin resistance. pancreatic insulin secretion increases to compensate: Hyperinsulinaemia
   
   • blood glucose: normal
   • insulin: high
2. hyperglycaemia occurs despite elevated insulin. Glucose tolerance is impaired (defined as glucose levels of 7.8 to 11.0 mM 2h after a 75g oral glucose tolerance test)
   
   • blood glucose: high
   • insulin: high
3. insulin secretion declines, probably due to death of β cells. High fasting blood glucose = diabetes
   
   • blood glucose: high
   • insulin: low

Question 21

What is the lipotoxicity model of insulin resistance?

Answer 21

• when caloric intake is excessive, adipocytes store increasing amounts of TAG until they can store no more
• enlarged adipocytes secrete MCP-1
• macrophages infiltrate adipose tissue in response to MCP-1
• macrophages in adipose tissue produce TNFalpha, which favours export of fatty acids
• adipocytes export fatty acids to muscle, where ectopic lipid deposits form
• ectopic lipid intergeres with GLUT4 movement to the myocyte surface, producing insulin resistance
• ectopic lipids might cause insulin resistance by interfering with insulin signalling pathways

Question 22

What is the insulin receptor?

Answer 22

• a pre-formed heterotetrameric receptor
• the alpha and beta subunits are joined covalently by disulphide bonds
• insulin binds to the extracellular alpha subunits and activates the transmembrane beta-subunits through a change in shape of the receptor
• the tyrosine kinase activity of each beta chain leads to trans-autophosphorylation of its partner’s tyrosine residues
• several tyrosines are phosphorylated in the intracellular domain of the beta chain

Question 23

What is the tyrosine kinase activity of the IR?

Answer 23

• protein tyrosine kinases are a family of enzymes that transfer the gamma-phosphate of ATP to specific tyrosine (Tyr, or Y) residues of the protein substrate (e.g. the IR)
• a receptor with tyrosine kinase activity is called a receptor tyrosine kinase or RTK (e.g. the IR)
• the reaction is a phosphorylation, an example of a post-translational modification

Question 24

What is IR signalling in target tissues?

Answer 24

• RTKs bind (i.e. “recruit”) intracellular signalling proteins
• the pYs of the IR interact with conserved domains of the recruited signalling proteins e.g. insulin receptor substrate 1 (IRS1) to initiate signalling events